A How could we produce a secondary beam of long neutral kaon

[aveline de grandpre]

how could we produce a secondary beam consisted of long neutral koans using PS or SPS ? I mean is there any example of such an experiment? what are the conditions for conducting this such as the type of the primary beam, the type of the target and the momentum of the
beam? I am interested to conduct an experiment on long neutral kaons decay in a fixed target experiment.

 

[mfb]

The kaon CP violation experiments used those beams. You will always have neutrons in it as well, but if you are fine with that just remove the charged objects and "wait" (distance) until everything short-living decayed.

 

[ChrisVer]

http://www.etap.physik.uni-mainz.de/267_ENG_HTML.php

 

[Vanadium 50]

. You will always have neutrons in it as well, but if you are fine with that just remove the charged objects and "wait"

And if you're not fine with that, that's kind of what you have to do anyway. Pretty much anything you do to reduce the neutrons also reduces the usable K0longs.

Im interested to conduct an experiment on long neutral kaons decay in a fixed target experiment.

Why CERN and not JPARC? Also, proposing an experiment is a pretty big deal. The PAC will look closely at your team and ask "can they do what the propose"?

 

[mfb]

And if you're not fine with that, that's kind of what you have to do anyway. Pretty much anything you do to reduce the neutrons also reduces the usable K0longs.

Proton-proton collisions below 6.5 GeV beam energy could work. No neutrons in the initial state, not enough energy to produce them as baryon pair, and the weak interaction is negligible.

K_L beams are probably something the CERN beamline for schools projects could use.

 

[Vanadium 50]

No neutrons in the initial state

Doesn't work. To get a K0 you need the reaction p + p --> p + Lambda + K0 + pi+; I need the Lambda to conserve baryon number. The Lambda can decay to n + pi. If I want to get rid of the Lambda by having two K mesons produced, e.g. p + p --> p + p + K0 + K0, that takes more energy, and p + p --> p + Lambda + K0 + pi+ will still go. But worse than this, you can have neutrons from reactions without kaons: p + p --> p + n + pi+, for example.

Furthermore, in a fixed target geometry, you need to build at least part of your target out of something other than hydrogen. So you can get neutrons by elastic scattering and spallation.

 

[mfb]

Ah right, the protons can become neutrons without the weak interaction.

Hydrogen gas as windowless target would be possible. I don't say it is good, but at least not impossible.

Anyway, neutrons live so long that their decays should be negligible in most cases.

 

[Vanadium 50]

The problem is not the neutron decay. It's that the neutron interacts with your detector.

Consider a very interesting and very difficult decay, like KL --> pi0 nu nubar. That has a SM branching fraction that's very, very tiny: like 10^-12. If you want to see the decay - which looks like two photons plus nothing - you need to beat down the background to well under one per trillion kaons. While it's hard for a neutron to fake a photon (or two to fake two), it's not one-in-a-trillion times hard. More like one in few a thousand. So the name of the game is to get the neutrons out of your sample.

This is hard. The least hard thing to do is to take advantage of the fact that neutrons tend to arrive late and interact more slowly, so you place tight timing cuts on the interaction. This works best when you kaons arrive in a small window and most of the beam spill is empty. But a mostly empty beam spill doesn't give you that many kaons. That's why i said there's this tension between beam purity and beam intensity.

KLOE gets a very pure sample by using e+e- annihilation to the phi(1020). The price they paid is that the number of events is measured in millions, so they don't see these ultra-rare decays.

 

[mfb]

ATLAS and CMS achieve a jet suppression of several thousands in their photon identification - and most of the remaining events are actually neutral pions decaying to two photons. I would expect a pion photon pair / neutron separation to be easier, and the detectors searching for rare kaon decays have a much cleaner environment.

The neutrons fly along the beam axis, the photons do not.

The pion leads to two photons close together in time.

Sure, you have to care about neutrons, but you can filter them out with much more than just the calorimeter response.

 

[Vanadium 50]

A great deal of the jet suppression comes from the lack of tracks. Purely calorimetrically you don't get as much. And unfortunately with neutrons, anything they can do, they will. Such as interact with residual gas and make a pizero. Now you have a perfectly good pizero that was not from your intended signal.

I'm looking at the KOTO proposal, and they get about 42 neutrons per Klong, and about 8 of them have energy above 1 GeV. It's a good document if you are interested in the troubles neutrons cause.